Sledgehammer-wielding robbers brutally assaulted a security guard at an exclusive Swiss watch store in Midtown yesterday before making off with a glittering haul worth $400,000.

In a brazen daylight heist, the thieves snatched watches and jewelry at Audemars Piguet at 40 E. 57th St.

Although just 10 pieces were grabbed, the robbers struck gold.

Sources said the most expensive watch taken was worth $140,000, with others priced in the tens of thousands.

The six robbers, one hiding a sledgehammer, walked into the store around midday.

They praised the store’s own-brand watches and mentioned their popularity with leading rappers.

But then chaos erupted.

The men, in their 20s and 30s, repeatedly punched the store’s security guard, smashed glass cases with the hammer, and scooped out their loot before fleeing.

Sources say the guard, a retired cop in his 50s, was armed – but his gun was wrested from him as he struggled with his assailants.

He escaped serious injury and was taken to New York Hospital, where he is in stable condition.

Police would not say how many people were in the store at the time, but no one else was injured.

Audemars Piguet spokesman Paul Lerner refused to confirm the value of the items taken but said the most costly watches, generally crusted with diamonds and other precious gems, retail for some $750,000; none sells for less than $5,000.

The secretive Swiss firm would not comment further. It even instructed employees at the store not to speak to the media.

“I can confirm only that a robbery took place and that my client is cooperating with police,” Lerner said.

The store has surveillance cameras and police are examining the footage.

Workers at nearby stores said they had previously beefed up security.

“People have come in here and tried to snatch bags,” said a security guard at Prada, across the street from Audemars Piguet.

“That’s why we installed locks on the doors.”

The guard said suspicious-looking customers are now locked out.

Anyone with information on the watch heist is asked to call Crime Stoppers at (800) 577-TIPS.